1. Field
The present invention relates to a redundancy technique for storing information, such as data and parities, to be stored across a plurality of storage.
2. Description of the Related Art
With storages getting larger, amounts of data stored in the storages have been increasing. For storages connected with severs, Redundant Arrays of Inexpensive Disks (RAID) technique is employed to protect important data.
RAID enables a bunch of storages such as hard disk drives to be treated as a single storage. There are seven RAID levels from RAID 0 to RAID 6 depending upon their properties such as performance speed and reliability. In situations having fault tolerance from two storage failures, RAID 6 has begun to receive attention recent years. In RAID 6, two sorts of parity are generated from data to be stored. The data and the parities are striped and stored across a plurality of storages.
Storage systems generally include storages of the same model that are manufactured at the same periods. Consequently, the storages included in the systems may fail at the same mean time between failures (MTBF). Therefore, if one of the storages fails, a sequential storage failure may occur within a short period of time. During rebuilding the storages in RAID, storages are vulnerable because of overheads associated therewith due to streams of data reading. Since the concurrent-failure-tolerance of RAID 6 vastly increases data protection over RAID 5, RAID 6 has been receiving increased attention. Data and parities stored in failed storages in a RAID group are rebuilt and stored by replacing the failed storages with spares storages called hot spare (HS) and by using data and parities stored in the rest of the storages. Hereinafter, the data and the parities stored in the storages are collectively referred to as information.
Conventional storage control device capable of rebuilding storages in RAID 6 has been disclosed in Japanese Laid-open patent publication No. 2006-259894. Where other storage fails during rebuilding data stored in a failed storage, data stored in the two failed storages are rebuilt coordinately depending on a rebuilding progress of the data having been rebuilt before the second failure occurs. Since the storage control device rebuilds only stripes not having been rebuilt after the second failure occurs, the redundant data are effectively rebuilt.
In addition to the two storage failures, one of hot spares that are replaced with the failed storages may fail during rebuilding the data stored in the two failed storages. Thus, RAID 6 may have a scheme tackling with the likelihood of the hot spare failure.